Didnt they just put out a greatest hits disk a couple of years ago? jeez.
Anyway, i think Greatest Hits packages are a good thing, especially if you just want to check out a band that you haven't heard much of before. If you decide you like 'em... then go get the rest of their cds.
What does bug me is when a band will release a greatest hits package and stick a "previously unreleased" track on it, just to get the hardcore fans to buy the disk.